There is a need for high strength, oxidation resistant and crack resistant cast alloys for use in internal combustion engine components such as exhaust manifolds and turbocharger housings and gas-turbine engine components such as combustor housings as well as other components that must function in extreme environments for prolonged periods of time. The need for improved high strength, oxidation resistant, crack resistant cast alloys arises from the desire to increase operating temperatures of diesel engines, gasoline engines, and gas-turbine engines in effort of increasing fuel efficiency and the desire to increase the warranted operating hours or miles for diesel engines, gasoline engines and gas-turbine engines.
Current materials used for applications such as exhaust manifolds, turbo-charger housings, and combustor housings are limited by oxidation and corrosion resistance as well as by strength at high temperatures and detrimental effects of aging. Specifically, current exhaust manifold materials, such as high silicon and molybdenum cast ductile iron (Hi—Si—Mo) and austenitic ductile iron (Ni-resist) must be replaced by cast stainless steels when used for more severe applications such as higher operating temperatures or when longer operating lifetimes are demanded due to increased warranty coverage. The currently commercially available cast stainless steels include ferritic stainless steels such as NHSR-F5N or austenitic stainless steels such as NHSR-A3N, CF8C and CN-12. However, these currently-available cast stainless steels are deficient in terms of tensile and creep strength at temperatures exceeding 600° C., do not provide adequate cyclic oxidation resistance for temperatures exceeding 700° C., do not provide sufficient room temperature ductility either as-cast or after service exposure and aging, do not have the requisite long-term stability of the original microstructure and lack long-term resistance to cracking during severe thermal cycling.
Currently, the corrosion-resistant grade of cast austenitic stainless steel, CN-12, is in commercial use for automotive applications but is not optimized for extended service applications (e.g. diesel applications). CN-12 provides adequate strength and aesthetics for automobiles for the anticipated life in comparison to cast iron, but lacks the improved creep resistance that is optimal when mounting turbo chargers (70 lbs.) onto diesel exhaust manifolds. Currently commercially available CN-12 austenitic stainless steel includes about 25 wt. % chromium, 13 wt. % nickel, smaller amounts of carbon, nitrogen, niobium, silicon, manganese, molybdenum and sulfur. The addition of sulfur is considered essential or desirable for machineability from the cast material. The amount of added sulfur ranges from 0.11 wt. % to 0.15 wt. %.
It is therefore desirable to have a steel alloy and articles made from a steel alloy that have improved strength at high temperatures and improved ductility for engine component applications requiring severe thermal cycling, high operation temperatures and extended warranty coverage.